10 plant diseases caused by fungi

10 plant

diseases

caused by fungi

introduction

Fungi are small, generally microscopic, plants lacking chlorophyll and conductive tissues. Most of the 100,000 fungus species known are strictly saprophytic, living on dead organic matter which they help decompose.

Some, about 50, species cause diseases in man, and about as many cause diseases in animals, most of them being superficial diseases of the skin or its appendages. More than 8000 species of fungi, however, can cause diseases in plants. All plants are attacked by some kinds of fungi, and each of the parasitic fungi can attack one or many kinds of plants. Some of the fungi can grow and multiply only by remaining in association with their host plants during their entire life (obligate parasites), others require a host plant for part of their life cycles but can complete their cycles on artificial media, and still others can grow and multiply on dead organic matter as well as on living plants (nonobligate parasites).

characteristics of

plant pathogenic fungi

MORPHOLOGY

Most fungi have a vegetative body consisting of more or less elongated, continuous filaments which may or may not have cross walls (septa). The body of the fungus is called mycelium, and the individual branches or 172 filaments of the mycelium are called hyphae (Fig. 33). Each hypha or

FIGURE 33.

Appearance of the vegetative body (mycelium) of two fungi in culture. (A) Physalospora. (B) Phoma.

mycelium may be uniform in thickness or may taper into thinner or broader portions. Hyphae of some fungi are only 0.5 μηι in diameter, while in others they may be more than 100 μηι thick. The length of the mycelium may be only a few microns in some fungi, but in others it may produce mycelial strands several meters long.

In some fungi the mycelium consists of cells containing one or two nuclei per cell. In others the mycelium is coenocytic, i.e., it contains many nuclei and either the entire mycelium is one continuous, tubular, branched or unbranched multinucleate cell, or it is partitioned by several septa, each segment being a multinucleate hypha. Growth of the mycelium occurs at the tips of the hyphae.

Some lower fungi lack true mycelium and produce instead a naked, amoeboid, multinucleate Plasmodium (e.g., Myxomycetes) or a system of strands of grossly dissimilar and continuously varying diameter called a rhizomycelium (e.g., Chytridiomycetes).

REPRODUCTION

Fungi reproduce chiefly by means of spores (Fig. 3 4 ) . Spores are specialized propagative or reproductive bodies consisting of one or a few cells. Spores may be formed asexually (i.e., through the separation of minute fragments of the mycelium into spores) or as the result of a sexual process.

In the lower fungi, asexual spores are produced inside a sac called a sporangium and are released through an opening of the sporangium or upon its rupture. Some of these spores are motile by means of flagella and are, therefore, called zoospores. Other fungi produce asexual spores called conidia by the cutting off of terminal or lateral cells from special hyphae called conidiophores. In some fungi terminal or intercalary cells of a

FIGURE 34.

Representative spores and fruiting bodies of the main groups of fungi.

PHYCOMYCETES ASCOMYCETEs]] IMPERFECT FUNGI BASIDIOMYCETES

hypha enlarge, round up, form a thick wall and separate to form chlamydospores. In still other fungi, asexual spores (conidia) are produced inside thick-walled structures called pycnidia.

Sexual reproduction, or processes resembling it, occur in most groups of fungi. In some, two cells (gametes) of similar size and appearance unite and produce a zygote, called a zygospore. In others, the gametes are of unequal size and the zygote which they form is called an oospore. In some fungi no definite gametes are produced, but instead one mycelium may unite with another compatible mycelium. In one group of fungi (Ascomy- cetes) the sexual spores, usually eight in number, are produced within the zygote cell, the ascus, and the spores are called ascospores. In another group of fungi (Basidiomycetes), sexual spores are produced on the outside of the zygote cell called the basidium and the spores are called basidio- spores.

For a large group of fungi (Fungi Imperfecti) no sexual reproduction is known either because they do not have one or because it has not yet been discovered. Apparently these fungi reproduce only asexually.

The union of the sexual nuclei in the zygote produces a diploid (2N) nucleus. Usually the first divisions of this nucleus are meiotic so that throughout its life the fungus contains haploid (IN) nuclei, except im- mediately after the union of the gamete nuclei. In some groups of fungi, however, especially in the Basidiomycetes and to a lesser extent in the Ascomycetes, the cells of the entire mycelium or of parts of the mycelium contain two haploid nuclei which remain separate inside the cell. Such mycelium is called dikaryotic but behaves very much as though it were a diploid mycelium (in which the two nuclei are united).

In most fungi both male and female gametes are produced on the same mycelium (hermaphroditic fungi). When the male gametes can fertilize the female ones of the same mycelium, the fungus is called homothallic.

In many cases, however, the male gametes can fertilize only the female gametes of another, sexually compatible mycelium, and the fungus then is called heterothallic.

ECOLOGY AND SPREAD

Almost all plant pathogenic fungi spend part of their lives on their host plants and part in the soil or on plant debris on the soil. Some fungi pass all of their lives on the host and only the spores may land on the soil where they remain inactive until they are again carried to a host on which they grow and multiply. Other fungi (e.g., ^Venturia) must pass part of their lives on the host as parasites and part on dead tissues on the ground as saprophytes in order to complete their life cycle in nature. The latter group of fungi, however, remain continually associated with host tissues, whether living or dead, and, in nature, do not grow on any other kind of organic matter. A third group of fungi grow parasitically on their hosts but they continue to live, grow, and multiply on the dead tissues of the host after its death, and may further move out of the host debris into the soil or other decaying plant material on which they grow and multiply as

strict saprophytes. The dead plant material which they colonize need not be related at all to the host they can parasitize. This group of fungi are usually soil pathogens, have a wide host range, and can survive in the soil for many years in the absence of their hosts. They too, however, need to infect a host from time to time in order to increase their populations, since protracted and continuous growth of these fungi as saprophytes in the soil results in more or less rapid reduction in their numbers.

During their parasitic phase fungi assume various positions in relation to the plant cells and tissues. Some fungi (e.g., powdery mildews) grow outside the plant surface but send their feeding organs (haustoria) into the epidermal cells of the plant. Some (e.g., ^Venturia) grow only between the cuticle and the epidermal cells. Others grow between the cells in the intercellular spaces and may or may not send haustoria into the cells.

Still others grow between and through the cells indiscriminately. Obli- gate parasites can grow only in association with living cells, being unable to feed on dead cells. On the other hand, the mycelium of some nonobli- gate parasites never comes in contact with living plant cells, because their enzymes macerate and kill the plant cells ahead of the mycelium. In most cases, however, regardless of the position of the mycelium in the host, the reproductive bodies (spores) of the fungus are produced at or very near the surface of the host tissues to ensure their prompt and efficient dissemination.

The survival and performance of most plant pathogenic fungi depend greatly on the prevailing conditions of temperature and moisture or the presence of water in their environment. Free mycelium survives only within a certain range of temperatures ( - 5 to +45°C) and in contact with moist surfaces, inside or outside the host. Most kinds of spores, however, can withstand broader ranges of both temperature and moisture and carry the fungus through the low winter temperatures and the dry summer periods. Spores, however, also require favorable temperatures and mois- ture in order to germinate. Furthermore, lower fungi producing zoospores require free water for the production, movement, and germination of the zoospores.

Zoospores are the only fungus structures that can move by themselves.

Zoospores, however, can move for only very short distances (a few mil- limeters or centimeters, perhaps). Besides, only some myxomycetes and some phycomycetes produce zoospores. The great majority of the plant pathogenic fungi depend for their spread from plant to plant and to different parts of the same plant on chance distribution by agents such as wind, water, birds, insects, other animals, and man. Fungi are dissemi- nated primarily in the form of spores. Fragments of hyphae and hard masses of mycelium known as sclerotia may also be disseminated by the same agents although to a much lesser extent.

Spore dissemination in almost all fungi is passive, although their initial discharge in some fungi is forcible. The distance to which spores may be disseminated varies with the agent of dissemination. Wind is probably the most important disseminating agent of spores of most fungi and may carry spores over great distances. For specific fungi, other agents such as water or insects may play a much more important role than wind in the dissemination of their spores.

classification of plant pathogenic fungi

The fungi that cause diseases on plants are a diverse group, and because of their large numbers and diversity, only a sketchy classification of some of the most important phytopathogenic genera will be presented here.

THE LOWER FUNGI

Class: MYXOMYCETES (The slime molds)—Lack mycelium. Their body is a naked, amorphous Plasmodium.

Order: Physarales—Saprophytic Plasmodium that gives rise to crusty fruc- tifications containing spores. They produce zoospores.

Genus: Fuligo, Mucilago, and Physarum cause slime molds on low-lying plants.

Order: Plasmodiophorales—Plasmodia produced within cells of roots and stems of plants. They produce zoospores.

Genus: Plasmodiophora, P. brassicae causing clubroot of crucifers.

Polymyxa, P. graminis being parasitic in wheat and other cere- als.

Spongospora, S. subterranea causing powdery scab of potato tubers.

Class: PHYCOMYCETES (Algal fungi, the lower true fungi).

Subclass: CHYTRIDIOMYCETES—Have round or elongated mycelium that lacks cross walls.

Order: Chytridiales—Have cell wall but lack true mycelium, at most a rhizomycelium. Zoospores.

Genus: Olpidium, O. brassicae being parasitic in roots of cabbage and other plants.

Physoderma, P. maydis causing brown spot of corn.

Synchytrium, S. endobioticum causing potato wart.

Urophlyctis, U. alfalfae causing crown wart of alfalfa.

Subclass: OOMYCETES (The water molds, white rusts, and downy mil- dews)—Have elongated mycelium. Produce zoospores in zoo- sporangia. Oospores produced by the union of morphologically different gametes.

Order: Saprolegniales—Have well-developed mycelium. Zoospores produced in long, cylindrical zoosporangia attached to mycelium. Oo- spores.

Genus: Aphanomyces, causing root rot of many vegetables.

Order: Peronosporales—Sporangia, usually zoosporangia produced at tips of hyphae and set free. Oospores.

Family: Pythiaceae

Genus: Pythium, causing damping off of seedlings, seed decay, root rots, and cottony blight of turf grasses.

Phytophthora, P. infestans causing late blight of potato, others causing mostly root rots.

Family: Albuginaceae (The white rusts)

Genus: Albugo, A. Candida causing white rust of crucifers.

Family: Peronosporaceae (The downy mildews)

Genus: Plasmopara, P. viticola causing downy mildew of grape.

Peronospora, P. nicotianae causing downy mildew (blue mold) of tobacco.

Bremia, B. lactucae causing downy mildew of lettuce.

Sclerospora, S. graminicola causing downy mildew of grasses.

Pseudoperonospora, P. cubensis causing downy mildew of cucurbits.

Subclass: ZYGOMYCETES (The bread molds)—Terrestrial fungi. Produce nonmotile asexual spores in sporangia. No zoospores. Their resting spore is a zygospore, produced by the fusion of two morphologically similar gametes.

Order: Mucorales—Produce zygospores. Nonmotile asexual spores formed in terminal sporangia.

Genus: Rhizopus, causing soft rot of fruits and vegetables.

Choanephora, C. cucurbitarum causing soft rot of squash.

THE HIGHER FUNGI

Class: ASCOMYCETES (The sac fungi)—Produce sexual spores, called asco- spores, in groups of eight within an ascus.

Subclass: HEMIASCOMYCETES—Asci naked, not in ascocarps.

Order: Taphrinales—Asci arising from binucleate ascogenous cells.

Genus: Taphrina—causing peach leaf curl, plum pocket, oak leaf blister, etc.

Subclass: EUASCOMYCETES—Asci produced in ascocarps.

Series: PYRENOMYCETES (The perithecial fungi)—Asci in fruiting bodies completely closed (cleistothecia) or in fruiting bodies with an opening (perithecia).

Order: Erysiphales (The powdery mildews)—Mycelium and cleistothecia on surface of host plant.

Genus: Erysiphe, causing powdery mildew of grasses, cucurbits, etc.

Microsphaera, one species causing powdery mildew of lilac.

Podosphaera, P. leucotricha causing powdery mildew of apple.

Sphaerotheca, S. pannosa causing powdery mildew of roses and peach.

Uncinula, U. necator causing powdery mildew of grape.

Order: Sphaeriales—Perithecia with dark-colored, usually firm walls.

Genus: Ceratocystis, C. ulmi causing the Dutch elm disease.

Diaporthe, causing bean pod blight, citrus melanose, and fruit rot of eggplant.

Endothia, E. parasitica causing chestnut blight.

Glomerella, G. cingulata causing many anthracnose diseases and bitter rot of apple.

Gnomonia, causing anthracnose or leaf spot diseases.

Rosellinia, causing root diseases of fruit trees and vines.

Valsa, causing canker diseases of peach and other trees.

Xylaria, causing tree cankers and wood decay.

Order: Hypocreales—Perithecia light-colored, or red or blue.

Genus: Claviceps, C. purpurea causing ergot of rye.

Gibberella, causing foot or stalk rot of corn and small grains.

Nectria, causing twig and stem cankers of trees.

Series: PSEUDOSPHAEROMYCETES (The ascostromatic fungi)—

Peritheciumlike stromata with asci in separate or single large cavities.

Order: Myriangiales—Cavities arranged at various levels and containing single asci.

Genus: Elsinoe, causing anthracnose of grape and raspberry, and scab of citrus.

Order: Dothideales—Cavities arranged in a basal layer and containing many asci. Perithecia lack pseudoparaphyses.

Genus: Dibotryon, D. morbosum causing block knot of cherries and plums.

Dothidella, D. ulei causing the leaf spot of rubber trees.

Guignardia, G bidwellii causing black rot of grapes.

Mycosphaerella, causing leaf spots of many plants.

Order: Pleosporales—Cavities arranged in a basal layer and containing many asci. Perithecia have pseudoparaphyses.

Genus: Ophiobolus, (Gaeumannomyces) causing the take-all disease of wheat.

Physalospora, P. obtusa causing black rot of apples.

Venturia, V. inaequalis causing apple scab.

Series: DISCOMYCETES (The cup fungi)—Asci produced at the surface of fleshy cup- or saucer-shaped apothecia.

Order: Helotiales—Asci release spores through an apical, circular perfora- tion.

Genus: Coccomyces, C. hiemalis causing cherry leaf spot.

Diplocarpon, D. rosae causing black spot of roses.

Lophodermium, causing pine needle blight.

Monilinia, M. fructicola causing brown rot of stone fruits.

Rhytisma, R. acerinum causing tar spot of maple leaves.

Sclerotinia, S. sclerotiorum causing watery soft rot of vegeta- bles.

Order: Pezizales—Ascospores released through cap- or lidlike structure at tip of ascus.

Genus: Pseudopeziza, P. medicaginis causing alfalfa leaf spot.

Class: IMPERFECT FUNGI OR DEUTEROMYCETES (Asexual fungi)—Sexual reproduction and structures lacking or unknown.

Order: Sphaeropsidales—Asexual spores produced in pycnidia.

Genus: Ascochyta, A. pisi causing pea blight.

Coniothyrium, causing cane blight on raspberry.

Cytospora, causing canker diseases on peach and other trees.

(sex. stage = Valsa)

Diplodia, D. zeae causing stalk and ear rot of corn.

Phoma, P. lingam causing black leg of crucifers.

Phomopsis, causing blights and stem cankers of trees.

Phyllosticta, causing leaf spots of many plants.

Septoria, S. apii causing late blight of celery.

Order: Melanconiales—Asexual spores produced in acervulus.

Genus: Colletotrichum, causing anthracnose on many field crops.

Coryneum, C. beijerincki causing blight on stone fruits.

Cylindrosporium, causing leaf spots on many kinds of plants.

Gloeosporium, similar if not identical to Colletotrichum, caus- ing anthracnose on many plants.

Marssonina, causing leaf and twig blight of poplar, strawberry leaf scorch, and anthracnose of walnuts.

Melanconium, M. fuligenum causing bitter rot of grape.

Sphaceloma, causing anthracnose of grape, raspberry, and scab of citrus and avocado.

Order: Moniliales—Asexual spores produced on or within hyphae freely exposed to the air.

Genus: Alternaria, causing leaf spots and blights on many plants.

Asperigillus, causing rots of stored seeds.

Botrytis, B. cinerea causing gray mold and blights on many plants.

Cercospora, one species causing early blight of celery.

Cladosporium, C. fulvum causing leaf-mold of tomato.

Fusarium, causing wilt and root rot diseases of many annual plants and cankers of forest trees.

Fusicladium, causing apple scab (sex. stage = Venturia).

Graphium, G. ulmi causing Dutch elm disease (sex. stage = Ceratocystis).

Helminthosporium, causing blight of cereals and diseases of turf grasses.

Penicillium, causing blue mold rot of fruits and other fleshy organs.

Phymatotrichum, P. omnivorum causing root rot of cotton and other plants.

Pyricularia, causing rice blast and gray leaf-spot of turf grasses.

Strumella, causing cankers on oak.

Thielaviopsis, T. basicola causing black root rot of tobacco.

Verticillium, causing wilt of many annuals and perennials.

Order: Mycelia Sterilia—No sexual or asexual spore forms common or known.

Genus: Rhizoctonia, causing root rots and crown rots of annals and brown-patch of turf grasses (Perfect stage Thanatephorus).

Sclerotium, causing root and stem rots of many plants (Perfect stage Pellicularia)

Class: BASIDIOMYCETES (The club fungi)—Sexual spores, called basidio- spores or sporidia, are produced externally on a one- or four-celled structure called a basidium.

Subclass: HETEROBASIDIOMYCETES (The rust and smut fungi)—

Basidium with cross walls or being the promycelium of a telio- spore. Teliospores single or united into crusts or columns, re- maining in host tissue or bursting through the epidermis.

Order: Ustilaginales—Fertilization by means of union of compatible spores, hyphae, etc. Only teliospores are produced.

Genus: Sphacelotheca, several species causing loose smut of sorghum.

Tilletia, several species causing bunt, or stinking smut, of wheat.

Urocystis, U. cepulae causing smut of onion.

Ustilago, causing smut of corn, wheat, barley, etc.

Order: Uredinales—Sperm cells called spermatia or pycniospores fertilize special receptive hyphae in spermagonia (pycnia). Produce aecio- spores, uredospores (repeating spores), teliospores, and basidio- spores.

Genus: Cronartium, C. ribicola causing white pine blister rust.

Gymnosporangium, G. juniperi-virginianae causing cedar apple rust.

Melampsora, M. lini causing rust of flax.

Phragmidium, one species causing rust of roses.

Puccinia, several species causing rust of cereals.

Uromyces, U. phaseoli causing rust of beans.

Subclass: HOMOBASIDIOMYCETES (The wood decay and root rot fungi)—Basidium without cross walls. Basidiocarp lacking or present. Include the mushrooms, shelf fungi, puff balls, etc.

Series: HYMENOMYCETAE—Basidia produced in a hymenium becom- ing exposed to the air before the spores are shot off from the sterigmata.

Order: Exobasidiales—Basidiocarp lacking: basidia produced on surface of parasitized tissue.

Genus: Corticium, one species causing the red thread disease of turf grasses.

Exobasidium, causing leaf, flower and stem galls on ornamen- tals.

Order: Polyporales—Hymenium lining the surfaces of small pores or tubes.

Genus: Eomes, causing heart rot of many trees.

Pellicularia (Sclerotium), causing root and stem rots of many plants.

Polyporus, causing root and stem rot of many trees.

Poria, causing wood and root rots of forest trees.

Stereum, causing wood decay and silver leaf disease of trees.

Thanatephorus, (Rhizoctonia) causing root and stem rots of many annual plants and brown patch of turf grasses.

Typhula, one species causing snowmold or blight of turf grasses.

Order: Agaricales—Hymenium on radiating gills or lamellae.

Genus: Armillaria, A. mellea causing root rots of forest and fruit trees.

Lenzites, causing brown rot of conifers and decay of wood prod- ucts.

Marasmius, causing the fairy ring disease of turf grasses.

Peniophora, causing decay of conifer logs and pulpwood.

Pholiota, causing brown wood rot in deciduous forest trees.

Pleurotus, causing white rot on many deciduous forest trees.

Schizophyllum, causing white rot in deciduous forest trees.

IDENTIFICATION

Since each fungus disease of plants is usually caused by only one fungus, and since there are more than 100,000 different species of fungi, the identification of the fungus species on a diseased plant specimen or culture of a fungus means that all but one of all the known fungus species must be excluded.

The most significant characteristics of a fungus used for identification are its spores and fructifications, or spore-bearing structures. These are examined under the compound microscope directly after removal from the specimen. The specimen is often kept moist for a few days to promote fructification development, or the fungus may be isolated and grown on artificial media, and identification is made on the basis of the fructifica- tions produced on the media.

The shape, size, color, and manner of arrangement of spores on the sporophores or the fruiting bodies, as well as the shape, color, etc. of the sporophores or fruiting bodies, are sufficient characteristics to suggest, to one somewhat experienced in the taxonomy of fungi, the class, order, family, and genus to which the particular fungus belongs. In any case, these characters can be utilized to trace the fungus through published analytical keys of the fungi to the genus and, finally, to the species to

which it belongs. Once the genus of the fungus has been determined, specific descriptions of the species are found in monographs of genera or in specific publications in research journals.

Since there are usually lists of the pathogens affecting a particular host plant, one may use such host indexes as short cuts in quickly finding names of fungus species that might apply to the fungus at hand. Host indexes, however, merely offer suggestions in determining identities, which must ultimately be determined by reference to monographs and other more specific publications.

SYMPTOMS CAUSED BY FUNGI ON PLANTS

Fungi cause local or general symptoms on their hosts and these may occur separately on different hosts, concurrently on the same host, or follow one another on the same host. In general fungi cause local or general necrosis or killing of plant tissues, hypertrophy and hypoplasia or stunting of plant organs or entire plants, and hyperplasia or excessive growth of plant parts or whole plants.

The most common necrotic symptoms are:

Leaf spots—Localized lesions on host leaves consisting of dead and collapsed cells.

Blight—General and extremely rapid browning of leaves, branches, twigs, and floral organs resulting in their death.

Canker—A localized wound or necrotic lesion, often sunken beneath the surface of the stem of a woody plant.

Root rot—Disintegration or decay of part or all of the root system of a plant.

Damping off—The rapid death and collapse of very young seedlings in the seed bed or field.

Basal stem rot—Disintegration of the lower part of the stem.

Soft rots and dry rots—Maceration and disintegration of fruits, roots, bulbs, tubers, and fleshy leaves.

Anthracnose—A necrotic and sunken ulcerlike lesion on the stem, leaf, fruit or flower of the host plant.

Scab—Localized lesions on host fruit, leaves, tubers, etc., usually slightly raised or sunken and cracked, giving a scabby appearance.

Almost all of the above symptoms may also cause pronounced stunt- ing of the infected plants. In addition, certain other symptoms such as leaf rust, mildews, wilts, and even certain diseases causing hyperplasia of some plant organs, such as clubroot, may cause stunting of the plant as a whole.

Symptoms associated with hypertrophy or hyperplasia and distortion of plant parts include:

Clubroot—Enlarged roots appearing like spindles or clubs.

Galls—Enlarged portions of plants usually filled with fungus mycelium.

Warts—Wartlike protuberances on tubers and stems.

Witches'-brooms—Profuse, upward branching of twigs.

Leaf curls—Distortion, thickening and curling of leaves.

In addition to the above, three groups of symptoms may be added:

Wilt—Usually a generalized secondary symptom in which leaves or shoots lose their turgidity and droop because of a disturbance in the vascular system of the root or of the stem.

Rust—Many, small lesions on leaves or stems, usually of a rusty color.

Mildew—Chlorotic or necrotic areas on leaves, stems, and fruit usually cov- ered with mycelium and the fructifications of the fungus.

In many diseases, the pathogen grows or produces various structures on the surface of the host. These structures, which include mycelium, sclerotia, sporophores, fruiting bodies, and spores, are called signs and are distinct from symptoms, which refer only to the appearance of infected plants or plant tissues. Thus, in the mildews, for example, one sees mostly the signs consisting of a whitish, downy growth of fungus mycelium and spores on the plant leaves, fruit, or stem/ while the symp- toms consist of chlorotic or necrotic lesions on leaves, fruit, and stem, reduced growth of the plant, etc.

isolation of fungi (and bacteria)

Most plant diseases can be diagnosed by observation with the naked eye or with the microscope and, for these, isolation of the pathogen is not necessary. There are many fungal and bacterial diseases, though, in which the pathogen cannot be identified because it is mixed with one or more contaminants, because it has not yet produced its characteristic fruiting structures and spores, because the same disease could be caused by more than one similar-looking pathogen and perhaps by some environmental factor, or because the disease is caused by a new, previously unknown pathogen that must be isolated and studied. }ust as often, pathogens of even known diseases must be isolated from diseased plant tissues whenever a study of the characteristics, habits, etc. of these pathogens is to be undertaken.

PREPARING FOR ISOLATION

Even before one attempts to isolate the fungus or bacterium pathogen from a diseased plant tissue, several preliminary operations must be performed. These include:

1. Sterilization of glassware, such as petri dishes, test tubes, pipettes, etc., by dry heat (150 to 160°C for 1 hour or more) or autoclaving or by dipping for 1 minute or more in a potassium dichromate-sulfuric acid solution, or in 1:1000 mercuric chloride, or 5 percent formalin, or 95 percent ethyl alcohol.

All chemically treated glassware should be rinsed through at least three changes of sterile (boiled or autoclaved) water.

2. Preparation of solutions for treating the surface of the infected or infested tissue so as to eliminate or markedly reduce surface contaminants that could interfere with the isolation of the pathogen. These solutions can be used either as a surface wipe or as a dip. The most commonly used surface sterilants include: 5.75 percent sodium hypochlorite (1 Clorox: 9 water) solution, used both for wiping infected tissues or dipping sections of such tissues in it and for wiping down table or bench surfaces before making isolations; 95 percent ethyl alcohol, which is mild and is used for leaf dips for 3 seconds or more; mercuric chloride 1:1000, for 15 to 45 seconds,- or mercuric chloride 1:1000 in 50 percent ethyl alcohol (Rada's solution). The tissues must be blotted dry with a sterile paper towel when treated with the first two solutions but they must be rinsed in 3 changes of sterile water when treated with the last two solutions.

3. Preparation of culture media on which the isolated fungal or bacterial pathogens will grow. An almost infinite number of culture media can be used to grow plant pathogenic fungi and bacteria. Some of them are entirely synthetic—made up of known amounts of certain chemical compounds—

and are usually quite specific for certain pathogens. Some are liquid or semiliquid and are used primarily for growth of bacteria but also of fungi in certain cases. Most media contain an extract of a natural source of carbohy- drates and other nutrients, such as potato, corn meal, lima bean, or malt extract, to which variable amounts of agar are added to solidify the medium and form a gel on or in which the pathogen can grow and be observed. The most commonly used media are potato dextrose agar (PDA), which is good for most, but not all fungi, water agar or glucose agar ( 1 - 3 percent glucose in water agar) for separating some fungi [Pythium and Fusarium) from bacteria, and nutrient agar, which contains beef extract and peptone and is good for isolating bacterial plant pathogens. Fungi can also be separated in culture from bacteria by adding 1 or 2 drops of a 25 percent solution of lactic acid, which inhibits growth of bacteria, to 10 ml of the medium before pouring it in the plate. Solutions of culture media are prepared in flasks which are then plugged and placed in an autoclave at 120°C and 15 pounds pressure for 20 minutes (Fig. 35). The sterilized media are then allowed to cool somewhat and are subsequently poured from the flask into sterilized petri dishes, test tubes, or other appropriate containers.

If agar was added to the medium, the latter will soon solidify and is then ready to be used for growth of the fungus or bacterium. The pouring of the culture medium into petri dishes, tubes, etc. is carried out as aseptically as possible either in a separate culture room or in a clean room free from drafts and dust. In either case, the work table should be wiped with a 10 percent Clorox solution, hands should be clean and tools such as scalpels, forceps or needles should be dipped in alcohol and flamed to prevent introduction of contaminating microorganisms.

It must be kept in mind that, of the different plant pathogens, the bacteria are the only ones whose members can all be grown on culture media. Although most fungi can be cultured on nutrient media with ease, some of them have specific and exacting requirements and will not grow on most commonly used nutrient media. Some groups of fungi, namely the Erysiphales, the causes of the powdery mildew diseases, and the Peronosporales, which cause the downy mildews, are considered strictly obligate parasites and cannot be grown on culture media. Another group of fungi, the Uredinales, which cause the rust diseases of plants, were,

nutrient mediu m

Distilled water (1/2 full )

Nutrient mediu m autoclave d

20' a t I5ps i Autoclaved mediu m Nutrien t mediu m i s allowed t o coo l poure d int o petr i dis h Preparation o f soli d nutrien t medi a i n plate s (petr i dishes )

Nutrient mediu m solidifies i n

petri dis h

Distilled water

Nutrient medi i boiled unti l dissolved

WW

w

Medium i s place d in funne l an d poured int o tube s

Tubes place d i n rac k and plugge d wit h cotton

Tubes autoclave d 2 0 '

at 1 5 ps i Tubes place d i n slanted positio n to solidif y Preparation o f soli d nutrien t medi a i n tes t tub e slant s

FIGURE 35.

Preparation of solid nutrient media in plates (petri dishes) and in test tube slants.

until recently, also thought to be strictly obligate parasites. In the last few years, however, it has become possible to grow in culture some stages of some rust fungi by the addition of certain components to the media and so the rust fungi are no longer considered to be obligate parasites. Of the other pathogens, only some spiroplasmas have been grown in culture.

None of the other 50 or so mycoplasmalike organisms and none of the rickettsialike bacteria, viruses, nematodes, or protozoa have been grown on nutrient culture media, so far, although it is expected that media for culturing mycoplasmalike organisms and rickettsialike bacteria will soon be discovered.

ISOLATING THE PATHOGEN FROM LEAVES

If the infection of the leaf is still in progress in the form of a fungal leaf spot or blight and if there are spores present on the surface, a few spores may be shaken loose over a petri plate containing culture medium or picked up at the point of a sterile needle or scalpel and placed on the surface of the culture medium. If the fungus does grow in culture, isolated colonies of mycelium will appear in a few days as a result of germination of the spores. These can be subcultured on separate plates and thus secure some plates that will contain the pathogen free of any contaminants.

Sometimes, isolation of the pathogen from fungal or bacterial leaf spots and blights is made by surface sterilizing the area to be cut with Clorox or Rada's solution, removing a small part of the infected tissue with sterile scalpel, etc., and placing it in a plate containing a nutrient medium.

The most common method, however, for isolating pathogens from infected leaves as well as other plant parts is the one in which several small sections 5- to 10-mm square are cut from the margin of the infected lesion so as to contain both diseased and healthy-looking tissue (Fig. 36).

These are placed in one of the surface sterilant solutions, making sure that the surfaces do get wet, and after about 15 to 30 seconds the sections are taken out aseptically one by one and at regular, e.g., 10- to 15-second intervals, so that each of them has been surface-sterilized for different times. The sections are then blotted dry on clean, sterile paper towels or are washed in three changes of sterile water, and are finally placed on the nutrient medium, usually three to five per dish. Those sections surface- sterilized the shortest time usually contain contaminants along with the pathogen, while those surface-sterilized the longest produce no growth at all because all organisms have been killed by the surface sterilant. Some of the sections left in the surface sterilant for intermediate periods of time, however, will allow only the pathogen to grow in culture in pure colonies, since the sterilant was allowed to act long enough to kill all surface contaminants but not too long to kill the pathogen which was advancing alone from the diseased to the healthy tissue. These colonies of

Infected plan t Section s fro m margi n o f Steril e forcep s Tissu e section s blotte d Section s ar e lesion place d i n 10 % cloro x use d t o transfe r wit h steril e pape r place d o n for differen t duration s section s towe l t o remov e nutrien t

clorox exces s mediu m i n

petri dis h

Sections place d o n I n correc t immersio n A pur e cultur e o f th e pathoge n i s obtaine d b y nutient medi a i n (e.g.90")onl y th e subculturin g a segmen t o f th e pathoge n growt h order o f immersio n pathoge n survive s i n i n th e previou s plat e int o a ne w plat e wit h time i n cloro x cente r o f sectio n an d nutrien t mediu m

grows ou t o f th e tissu e FIGURE 36.

Isolation of fungal pathogens from infected plant tissue.

the pathogen are then subcultured aseptically for further study of the pathogen.

If fruiting structures (pycnidia, perithecia) are present on the leaf, it is sometimes possible to pick them out, drop them in the surface sterilant for a few seconds, and then plate them on the nutrient medium. This procedure, however, requires that most of the work be done under the stereoscopic microscope (binoculars) since the fruiting structures are generally too small to see and to handle with the naked eye. Fruiting structures, after surface sterilization, may also be crushed in a small drop of sterile water and then the spores in the water are diluted serially in small tubes or dishes containing sterile water. Finally, a few drops from each tube of the serial dilution are placed on a nutrient medium and single colonies free of contaminants develop from germinating spores obtained from some of the serial dilution tubes.

The serial dilution method is often used to isolate pathogenic bacteria from diseased tissues contaminated with other bacteria. After surface sterilization of sections of diseased tissues from the margin of the infec- tion, the sections are ground aseptically but quite thoroughly in a small volume of sterile water, and then part of this homogenate is diluted serially in equal volumes or ten times the volume of the initial water.

Finally, plates containing nutrient agar are streaked with a needle or loop dipped in each of the different serial dilutions and single colonies of the pathogenic bacterium are obtained from the higher dilutions that still contain bacteria.

FROM STEMS, FRUITS,

AND OTHER AERIAL PLANT PARTS

Almost all the methods described for isolating fungal and bacterial patho- gens from leaves can also be used to isolate these pathogens from super- ficial infections of the above tissues. In addition to these methods, how- ever, pathogens can be often isolated easily from infected stems, fruits, etc., in which the pathogen has penetrated fairly deeply, by splitting the stem or breaking the fruit from the healthy side first and then tearing it apart toward and past the infected margin, thus exposing tissues not previously exposed to contaminants and not touched by hand or knife and therefore not contaminated. Small sections of tissue can be cut from the freshly exposed area of the advancing margin of the infection with a flamed scalpel and can be plated directly on the culture medium.

FROM ROOTS, TUBERS,

FLESHY ROOTS, VEGETABLE FRUITS IN CONTACT WITH SOIL, ETC.

Isolating pathogens from any diseased plant tissue in contact with soil presents the additional problems of numerous saprophytic organisms invading the plant tissue after it has been killed by the pathogen. For this reason, repeated, thorough washing of such diseased tissues to remove all soil and most of the loose, decayed plant tissue, in which most of the saprophytes are present, is the first step in isolating the pathogen. If the

infected root is small, once it is washed thoroughly pathogens can be isolated from it by following one of the methods described for isolating pathogens from leaves. If isolation is attempted from fleshy roots or other fleshy tissues and penetration of the pathogen is slight resulting only in surface lesions, the tissue is washed free from adhering soil, and several bits of tissue from the margin of the lesions are placed in Clorox or Rada's solution. The tissue sections are picked from the solution one by one, blotted or washed in sterile water, and placed on agar in petri plates.

If the pathogen has penetrated deeply into the fleshy tissue, the method described above for stems and fruit, i.e., by breaking the specimens from the healthy side first, then tearing toward the infected area and plating bits taken from the previously unexposed margin of the rot, can be used most effectively.

life cycles of fungi

Although the life cycles of the fungi of the different groups vary greatly, the great majority of them go through a series of steps that are quite similar (Fig. 37). Thus almost all fungi have a spore stage with a simple, haploid (possessing one set of chromosomes or IN) nucleus. The spore germinates into a hypha which also contains haploid nuclei. The hypha may either produce simple, haploid spores again (as is always the case in the Imperfect Fungi) or it may fuse with another hypha to produce a fertilized hypha in which the nuclei unite to form one diploid nucleus, called zygote (containing two sets of chromosomes, or 2N). In the Phycomycetes the zygote will divide to produce simple, haploid spores which close the cycle. In a brief phase of most Ascomycetes, and gener­

ally in the Basidiomycetes, the two nuclei of the fertilized hypha do not unite, but remain separate within the cell in pairs (dikaryotic or Ν + Ν) and divide simultaneously to produce more hyphal cells with pairs of nuclei. In the Ascomycetes, the dikaryotic hyphae are found only inside the fruiting body, in which they become the ascogenous hyphae, since the two nuclei of one cell of each hypha unite into a zygote [2N] which divides meiotically to produce ascospores that contain haploid nuclei.

In the Basidiomycetes haploid spores produce only short haploid hyphae. Upon fertilization, dikaryotic [Ν + N) mycelium is produced and this develops into the main body of the fungus. Such dikaryotic hyphae may produce, asexually, dikaryotic spores that will grow again into a dikaryotic mycelium. Finally, however, the paired nuclei of the cells unite and form zygotes. The zygotes divide meiotically and produce basidiospores that contain haploid nuclei.

In the Imperfect Fungi, of course, only the asexual cycle (haploid spore -» haploid mycelium —> haploid spore) is found. Even in the other fungi, however, a similar asexual cycle is the most common one by far, since it can be repeated many times during each growth season. The sexual cycle usually occurs only once a year.

S T E R I L E F U N G I

Germinating conidiu m

I M P E R F E C T

F U N G I Sporangiospor e

^ ( o r zoospore )

Sporangium

B A S I D I O M Y C E T E S

P H Y C O M Y C E T E S

A S C O M Y C E T E S

Fertilization Dikaryotic myceliu m

Sporangium Sporangiospore (or zoospore ) Ascus

FIGURE 37.

Schematic presentation of the generalized life cycles of the main groups of phytopathogenic fungi.

control of fungus diseases of plants

The endless variety and the complexity of the many fungus diseases of plants have led to the development of a correspondingly large number of approaches for their control. The particular characteristics in the life cycle of each fungus, its habitat preferences and its performance under certain environmental conditions are some of the most important points to be considered in attempting to control a plant disease caused by a fungus. Although some diseases can be controlled completely by just one type of control measure, a combination of measures is usually necessary for satisfactory control of most diseases.

The use of pathogen-free seed or propagating stock is always recom- mended and, for control of certain diseases, it is mandatory. Destruction of plant parts or refuse harboring the pathogen, destruction of volunteer plants or alternate hosts of the pathogen, use of clean tools and contain- ers, proper drainage of fields and aeration of plants, are all very important

practices in the control of most plant diseases caused by fungi. Crop rotation is helpful in controlling diseases caused by some fungi, but does not satisfactorily control fungi that have wide host ranges, can live saprophytically for a long time, or produce long-lived resting spores.

The use of plant varieties resistant to certain pathogens has found its greatest application in controlling fungus diseases of plants. Some of the most serious fungus diseases (e.g., rusts, Fusarium wilts) of the most important crop plants, are successfully controlled today by the use of resistant varieties. Although the degree of control through resistant va­

rieties varies with the crop and the pathogenic fungus involved, its suc­

cesses as of this time and the very low overall cost of such control make this type of control the most promising for the future.

The most effective method, however, and, sometimes the only one available for controlling most of the fungus diseases of plants, is through application of chemical sprays or dusts on the plants, their seeds, or into the soil where the plants are to grow. Soil-inhabiting fungi may be controlled in small areas by steam or electric heat, and in somewhat larger areas by volatile liquids, such as formaldehyde, chloropicrin, methyl bromide. Some diseases caused by soil-inhabiting fungi can also be controlled, and at a much lower cost, by applying fungicides on the seeds or other propagating materials, such as tubers and corms. Such treatment will also protect the seed from mycelium or spores carried on the seed. Fungicides used for seed treatment include, among others, carboxin, chloroneb, chloranil, dichlone, captan, and thiram.

Most fungicides are used to prevent diseases on the aboveground parts of plants and are applied on the foliage as sprays or dusts. Most of these are protectant, since they can only prevent fungi from causing infection, but cannot stop an infection once it has started. The number of such fungicides is great and includes many inorganic and organic compounds.

In recent years several systemic fungicides have been developed, and their use and effectiveness are increasing steadily. In addition to these, certain antibiotics (e.g., cycloheximide) are also effective against certain fungus diseases of plants.

In some diseases (e.g., loose smuts of cereals) the fungus is carried in the seed and control can be obtained only through treatment of the seed with systemic fungicides or hot water. In others, control of the insect vectors may be the only available possibility. In general, great advances have been made toward controlling fungus diseases of plants, especially through resistant varieties and through chemicals, and as a result, these diseases are probably much easier to control than any other group of plant diseases, although the losses caused by fungus diseases of plants are still very great.

SELECTED REFERENCES

Ainsworth, G. C , et al (eds.). 1 9 6 5 - 1 9 7 3 . "The Fungi. An Advanced Treatise."

Vols. 1 - 4 . Academic Press, New York.

Alexopoulos, C. J. 1962. "Introductory Mycology," Wiley, New York. 613 pp.

Barnett, H. L., and Β. B. Hunter. 1972. "Illustrated Genera of Imperfect Fungi,"

Burgess, Minneapolis, Minnesota. 218 pp.

Barnett, H. L., and F. L. Binder. 1973. The fungal host-parasite relationship. Ann.

Rev. Phytopathol. 1 1 : 2 7 3 - 2 9 2 .

Clements, F. E., and C. L. Shear. 1957. ' T h e Genera of Fungi." Hafner, New York.

496 p. 58 pi.

Cummins, G. B. 1959. "Illustrated Genera of Rust Fungi." Burgess, Minneapolis, Minn. 131 p.

Fergus, C. L. 1960. "Illustrated Genera of Wood Destroying Fungi," Burgess, Minneapolis, Minnesota. 132 p.

Griffin, D. M. 1969. Soil water in the ecology of fungi. Ann. Rev. Phytopathol.

7 : 2 8 9 - 3 1 0 .

Kendrick, B. (Ed.). 1971. "Taxonomy of Fungi Imperfecti." Toronto Univ. Press, Toronto. 309 pp.

Meredith, D. S. 1973. Significance of spore release and dispersal mechanisms in plant disease epidemiology. Ann. Rev. Phytopathol. 1 1 : 3 1 3 - 3 4 2 .

Stevens, F. L. 1913. "The Fungi Which Cause Plant Disease," Macmillan, New York. 754 p.

Tsao, P. H. 1970. Selective media for isolation of pathogenic fungi. Ann. Rev.

Phytopathol. 8 : 1 5 7 - 1 8 6 .

diseases caused by the lower fungi

DISEASES CAUSED BY MYXOMYCETES Myxomycetes are fungi whose vegetative body is a Plasmodium, i.e., an

amoeboid mass of protoplasm that has many nuclei and no definite cell wall. In the true Myxomycetes, also called slime molds, the Plasmodium does not invade plant cells and is used up to form superficial fructifica- tions that contain resting spores. In another group of fungi, the Plas- modiophorales, the vegetative body is also a Plasmodium but it is pro- duced only in the cells of the host plant and their resting spores are produced in masses but not in distinct fructifications. Both the true slime molds and the Plasmodiophorales produce zoospores that usually have two flagella (Fig. 38).

Two groups of Myxomycetes cause diseases of plants, the one by simply growing externally on the surface of leaves, stems and fruits without parasitizing the plant, and the other by entering and parasitizing the roots and other below-ground parts of plants (Fig. 39). These groups are:

I. Physarales. It includes the true slime mold genera Fuligo; Mucilago, Physarum, and others, which cause slime molds on the surface of low-lying plants.

II. Plasmodiophorales. It includes three obligate plant parasitic genera: Plas- modiophora, Polymyxa, and Spongospora.

THE TRUE SLIME MOLDS

Slime molds appear on plants growing low on the ground, such as turf grasses, strawberries, vegetables, and small ornamentals (Fig. 40). They

^ THE LOWE^FONGI ^ ^^^^sarum ^ PlasmodLpliora ^ Polymyxa Spongosporo ° Olpidium ^ ^ ^ Physoderma ^ Synchy^ium ^UrQPh|yctls > Aphanomyces ^ Pythium ^ ^ ^ Phytophthora Albugo Plasmopara ^ Bremia Peronosporo , Pseudoperonospora| Sclerosporo

f

FIGURE 38.

The most common Lower Fungi (Myxomycetes and Phycomycetes) that cause disease in plants: a—antheridium, gs—germinating sporangium, h—haustorium, m—mycelium, og—oogonium, os—oospore, ρ—Plasmodium, pws—pustule with sporangia, rm—rhizomycelium, rs—resting spore, rsa—resting sporangium, s—sporangium, sp—sporangiophore, ss—sporangiospore, th—thallus, ζ—zoospore, zs—zoosporangium, zy—zygospore.

FIGURE 39.

The most common symptoms caused by Myxomycetes and Phycomycetes.

FIGURE 40.

(A) Begonia leaf covered with fructifications (sporangia) of a slime mold. (B) Slime mold fructifications on the blades of a turfgrass.

are most common in warm weather following heavy rains or watering.

All aboveground parts of plants in some areas, and even the soil between plants, may be covered by a creamy white or colored slimy growth which later changes to distinct, crusty, ash-gray, or colored fruiting structures.

The latter give the affected plants a dull gray appearance.

Slime molds are saprophytic members of the true slime molds (Myxomycetes). Their plasmodium creeps like an amoeba and feeds on decaying organic matter and microorganisms such as bacteria which it simply engulfs and digests. There are many species of slime mold fungi, the most common of which are Physarum, Fuligo, and Mucilago.

The plasmodium grows mostly in the upper layer of the soil and in the thatch, but during or after warm wet weather it comes to the soil surface and creeps over low-lying vegetation. On the plant surface, which these fungi use merely as a means of support, the plasmodium produces the crusty fruiting structures which vary in size, shape and color depending on the species of slime mold (Figs. 40 and 41). The fruiting structures are sporangia, i.e., containers filled with dark masses of powdery spores, and are easily rubbed off the plant. The spores are spread by wind, water, mowers or other equipment and can survive unfavorable weather. In cool, humid weather, the spores absorb water, their cell wall cracks open and a

Plasmodium an d surfac e an d o n lea f fructifications

on lea f FIGURE 41.

Life cycle of slime mold fungi.

single, naked, motile swarm spore emerges from each. The swarm spores feed like the Plasmodium while they undergo several divisions and vari­

ous changes. Finally they unite in pairs to form amoeboid zygotes which enlarge, become multinucleate and become the Plasmodium.

No control is usually considered necessary against slime molds. When they become too numerous and unsightly, breaking up of the spore masses by raking, brushing, or hosing down with water in dry weather, and removal of affected leaves or mowing of grass corrects the problem.

Slime mold fungi are generally very sensitive to many fungicides, so if the problem becomes serious, spraying with any fungicide, such as captan, thiram, etc., used to control other diseases of the particular plant should also control the slime molds.

SELECTED REFERENCES

Alexopoulos, C. J. 1962. "Introductory Mycology" (2nd ed.). John Wiley and Sons, Inc., New York. 613 p.

Couch, Η. B. 1973. "Diseases of Turfgrasses" (2nd ed.). Krieger Publishing Co., New York. 348 p.

DISEASES OF

BELOW-GROUND PLANT PARTS CAUSED BY MYXOMYCETES

Three common and often severe diseases of below-ground parts of plants are caused by Myxomycetes of the order Plasmodiophorales. The fungi involved are:

Plasmodiophora, causing clubroot of crucifers

Polymyxa, causing a root disease of cereals and grasses Spongospora, causing the powdery scab of potato (Fig. 42) These fungi are widespread in soils in which they overwinter as resting spores. When the temperature is favorable and moisture is plenti­

ful, the resting spore produces one zoospore which infects a root hair and produces a Plasmodium. The latter is transformed into zoosporangia which produce numerous secondary zoospores which, probably after pair­

ing, enter root or tuber tissues, produce Plasmodium and cause the typical disease. The Plasmodium spreads into the host tissues and is finally transformed into overwintering resting spores.

The pathogens are obligate parasites and, although they can survive in the soil as resting spores for many years, they can only grow and multiply in a rather limited number of hosts. The Plasmodium lives off the host cells it invades and it does not kill these cells. On the contrary, in some diseases many invaded and adjacent cells are stimulated by the pathogen to enlarge and divide, thus making available more nutrients for the pathogen. These pathogens spread from plant to plant by means of zoo­

spores and by anything that moves soil or water containing spores, by infected transplants, etc. Control of these pathogens is difficult and de­

pends mostly on avoiding contamination of pathogen-free soils, use of healthy transplants, tubers, etc., crop rotation with nonhost plants, ad­

justment of soil pH, etc.

FIGURE 42.

Mature stage of powdery scab of potato caused by Spongospora subterranea.

Polymyxa and Spongospora, in addition t o t h e diseases t h e y cause, can also t r a n s m i t d e s t r u c t i v e plant viruses, Polymyxa being a v e c t o r of soil- borne w h e a t m o s a i c virus and Spongospora of t h e p o t a t o m o p top virus.

• Clubroot of Crucifers

T h e clubroot disease of cruciferous plants, such as cabbage and cauliflower, is widely distributed all over t h e world, found w h e r e v e r plants of t h e m u s t a r d family grow. It has been observed m o s t frequently in Europe and N o r t h A m e r i c a .

Clubroot c a u s e s serious losses w h e n susceptible varieties of a n y cruciferous species are g r o w n in infested fields. Fields o n c e infested w i t h t h e clubroot pathogen r e m a i n s o indefinitely and b e c o m e unfit for culti- v a t i o n of crucifers.

Symptoms. Infected plants m a y h a v e pale green t o y e l l o w i s h leaves w h i c h m a y s h o w flagging a n d wilting in t h e middle of hot, s u n n y days but m a y r e c o v e r during t h e night (Fig. 4 3 A ) . Affected plants s h o w a l m o s t n o r m a l vigor a t first, b u t t h e n gradually b e c o m e stunted. Y o u n g plants m a y be killed by t h e disease w i t h i n a short t i m e after infection, w h i l e older plants m a y r e m a i n alive b u t fail t o p r o d u c e m a r k e t a b l e heads.

T h e m o s t c h a r a c t e r i s t i c s y m p t o m s of t h e disease appear o n t h e roots and s o m e t i m e s t h e underground part of t h e s t e m (Fig. 4 3 B ) . T h e s y m p - t o m s consist of s m a l l o r large spindlelike, spherical, knobby, or club- shaped swellings o n t h e roots and r o o t l e t s . T h e s e m a l f o r m a t i o n s m a y be isolated and c o v e r only part of s o m e roots o r t h e y m a y c o a l e s c e and c o v e r the entire root s y s t e m of t h e plant. T h e older a n d usually t h e larger clubbed roots disintegrate before t h e end of t h e s e a s o n due t o invasion by bacteria a n d o t h e r w e a k l y parasitic soil m i c r o o r g a n i s m s .

FIGURE 43.

(A) Midday wilting of half-grown cabbage plants that have severely clubbed roots.

(B) Malformed, spindlelike or clubbed roots infected with Plasmodiophora brassicae.

The pathogen: Plasmodiophora brassicae. It is a slime mold, the body of which is a Plasmodium. The Plasmodium gives rise to zoosporan- gia or to resting spores. Upon germination they produce zoospores. The single zoospore from resting spores penetrates host root hairs and there develops into a Plasmodium. After a few days, the Plasmodium cleaves into multinucleate portions surrounded by separate membranes; each portion develops into a zoosporangium. The zoosporangia are discharged outside the host through pores dissolved in the host cell wall, and each zoosporangium releases four to eight secondary zoospores. Some of these zoospores fuse in pairs to produce zygotes which can cause new infec- tions and produce new Plasmodium. The Plasmodium finally turns into resting spores (Fig. 4 4 ) which are released into the soil upon disintegra- tion of the host cell walls by secondary microorganisms.

Development of disease. The Plasmodium resulting from the germi- nation of the secondary zoospores penetrates young root tissues directly;

it can also penetrate secondarily thickened roots and underground stems through wounds. From these points of primary infection the Plasmodium spreads to cortical cells and reaches the cambium through direct penetra- tion of host cells (Fig. 45). From the point of infection of the cambium the Plasmodium spreads in all directions in the cambium, outward into the cortex and inward toward the xylem and into the medullary rays. Single- point infections result in spindle-shaped clubs, being widest at the point of invasion and tapering off away from it.

As the plasmodia pass through cells, they become established in some of them and stimulate these cells to abnormal enlargement (hypertrophy) and abnormal division (hyperplasia). Infected cells may be five or more

FIGURE 44.

Scanning electron micrograph of resting spores of Plasmodiophora brassicae within cells of club roots. (Photo courtesy M. F. Brown and H. G. Brotzman)

xlOOO.

times larger than adjacent uninfected ones. The infected cells of a club are distributed in small groups throughout the diseased tissue and the groups are usually separated by uninfected cells. The stimulus which is respon- sible for the abnormal growth of the cells appears to diffuse in advance of the pathogen and acts on the noninvaded cells of diseased tissues as well as on the infected ones. Actively growing and dividing cells, i.e., cambial cells, are more easily invaded by the pathogen and are more responsive to the stimulus than other cells.

In most cases many cells of infected clubs remain free from plasmodia, but in rare instances almost all the cells of a club may be infected. When few cells are infected the plasmodia become large, whereas when many cells are infected, they remain relatively small. Thus, there seems to be a fairly constant ratio between the volume occupied by the plasmodium